Qualitative and quantitative evaluation of particles and microorganisms in air is important in numerous fields such as: the pharmaceutical industry; the agrifood industry; medicine; sanitary services; veterinary services, etc. The dimensions of the particles and microorganisms for collection may lie in the range about 0.5 micrometers (μm) to about 15 μm.
The biological particles present in air comprise in particular bacteria, molds, viruses, pollens, etc., and they are generally collected by impact on a nutritive or selective agar-agar culture medium, or by impact on a liquid surface, or indeed by centrifuging in a liquid medium.
The collecting of such particles or microorganisms by impact on agar-agar is followed by incubation, and the colonies that are obtained by culturing are counted and identified. A drawback of that method is that only those microorganisms that are viable and cultivatable on the medium used can be counted, while it is not possible to count microorganisms that are already stressed on being collected or that become stressed as the result of being sucked in or made to impact against agar-agar.
That method can therefore be implemented using only very low air suction flow rates and for short lengths of time in order to avoid damaging the collected microorganisms, and also to avoid drying out the agar-agar, with said air flow rate typically being less than 100 liters per minute (L/min). The microbial load measured with that method is generally under-evaluated.
When collecting by impact on a liquid surface, the particles present in the sucked-in air are projected against wet walls of a chamber and they are recovered at the bottom of the chamber in the form of a liquid sample. a drawback with that method is the bubbling of the liquid caused by the flow of air and the re-dispersion of particles by bouncing against the walls of the chamber.
When collecting by centrifuging in a liquid medium, the particles are sucked into a cylindrical chamber where they are set into rotation and then fall into a tube containing a liquid. The largest particles are pressed against the walls of the chamber and recovered from the bottom, while the finest particles are carried towards the outlet by the air flow and are not collected. In existing systems, the air flow rates used are relatively high in order to ensure that sufficient particles and microorganisms are collected, and the dimensions and the weight of the equipment used (cyclone chamber, air suction means) are relatively large so that the equipment is stationary and installed on a permanent basis, which restricts possible uses thereof.
In addition, the collected particles are generally recovered in a flask at the bottom end of the cylindrical chamber, and the flask is then closed by means of a stopper. That way of proceeding firstly fails to ensure that all of the separated particles are indeed collected in the flask, and secondly does not avoid risks of contamination while the flask is being separated from the cyclone chamber.